Calcium-silicate based slag for treatment of molten silicon

ABSTRACT

The present invention relates to a calcium-silicate based slag having a phosphorous content of less than 3 ppmw. The invention further relates to a method for producing a low phosphorous calcium-silicate based slag, where molten calcium-silicate based slag is treated with a molten ferrosilicon alloy in a vessel, whereby phosphorous in the calcium-silicate based slag is transferred to the ferrosilicon alloy, and a low phosphorous calcium-silicate to based slag is removed from the vessel.

RELATED APPLICATION

This application is a divisional application of the U.S. applicationSer. No. 10/515,583, which is a 371 of PCT/NO03/00091.

FIELD OF INVENTION

The present invention relates to a calcium-silicate based slag having avery low phosphorous content, a method for producing suchcalcium-silicate based slag having a very low phosphorous content, andto the use of the calcium-silicate based slag for the removal of boronand phosphorous from molten silicon.

BACKGROUND ART

In the method of refining metallurgical grade silicon in order toproduce pure silicon for solar cell production (solar grade silicon) itis known to remove boron by treatment of molten silicon with acalcium-silicate based slag. Such a method is disclosed in U.S. Pat. No.5,788,945. In order to remove boron from silicon to an acceptable lowlevel it is necessary to use a slag having a low boron content.

For solar grade silicon there is also strict requirements to the contentof phosphorous. Thus the phosphorous content of solar grade siliconshould be less than 3 ppmw.

The slag treatment of silicon in order to remove boron also influencesthe phosphorous content of silicon. Thus the distribution coefficientbetween phosphorous in slag and phosphorous in silicon is very low andin the range between 0.1 and 0.3. If calcium-silicate based slag used toremove boron from silicon contains too much phosphorous, the phosphorouscontent of silicon can therefore be increased during the slag treatment.It is thus important to use a low phosphorous containingcalcium-silicate based slag for removal of boron from silicon.

When preparing phosphorous containing calcium-silicate based slag fromCaO and Si0₂ it is difficult to find a CaO source having a sufficientlylow phosphorous content. In order not to increase the phosphorouscontent of silicon during slag treatment the calcium-silicate basedslag, the phosphorous content of the calcium-silicate based slag shouldbe as low as possible and preferably well below 3 ppmw.

DISCLOSURE OF INVENTION

It is an object of the present invention to provide a calcium-silicatebased slag having a very low phosphorous content and a method forremoving phosphorous from calcium-silicate based slag where low costsources of CaO and Si0₂ can be used to produce the slag.

Thus, according to a first aspect the present invention relates to acalcium-silicate based slag for treatment of molten silicon, said slaghaving a phosphorous content of less than 3 ppmw.

According to a preferred embodiment the calcium-silicate based slag hasa phosphorous content below 1 ppmw.

According to a preferred embodiment the calcium-silicate based slagcontains CaF₂ and/or MgO in an amount of up to 30% by weight in order tolower the viscosity of the slag and to increase removal of phosphorousand boron from silicon.

According to another preferred embodiment the calcium-silicate basedslag contains one or more of BaF₂, BaO, LiF and Li₂O in an amount of upto 10% by weight in order to adjust the density of the slag tofacilitate removal of the slag from silicon after slag treatment.

According to yet another preferred embodiment the calcium-silicate basedslag contains Al₂0₃ in order to be able to adjust the aluminum contentof silicon to be treated with the slag.

According to a second aspect the present invention relates to a methodfor producing a low phosphorous calcium-silicate based slag, whichmethod is characterized in that molten calcium-silicate based slag istreated with a molten ferrosilicon alloy in a vessel whereby phosphorousin the calcium-silicate based slag is transferred to the ferrosiliconalloy, and separating the molten low phosphorous calcium-silicate basedslag from the molten ferrosilicon alloy.

According to a preferred embodiment a molten layer of a ferrosiliconalloy is s provided in the vessel, whereafter a Si0₂ source, a CaOsource are supplied to the top of the layer of ferrosilicon alloy toprovide a layer of molten calcium silicate based slag wherebyphosphorous in the calcium-silicate based slag is transferred to theferrosilicon alloy, and removing the low phosphorous calcium-silicatebased slag from the vessel.

Preferably the ferrosilicon alloy comprises up to 30% by weight silicon,the remaining being iron except for normal amounts of impurities. Morepreferably the ferrosilicon alloy comprises 10-20% by weight silicon.The amount of silicon in the ferrosilicon alloy should balance theamount of Si02 in the liquid calcium-silicate based slag in order toprevent silicon in the slag from entering the ferrosilicon alloy andthus changing the composition of the slag.

According to another embodiment, the molten ferrosilicon alloy can beproduced in situ by adding Fe203 and Si together with the slag formingcompounds Si0₂ and CaO. Upon heating Fe203 will be reduced to Fe by someof the Si added and form the ferrosilicon alloy. In this embodiment theamount of Si02 supplied may have to be adjusted in order to compensatefor the amount of Si02 produced when Fe203 is reduced by Si, in order toobtain a suitable final composition of the calcium-silicate based slag.

In order to increase the speed of reaction between the moltencalcium-silicate based slag and the molten ferrosilicon alloy a reducingand/or an inert gas or a mixture of such gases are supplied to thevessel in order to stir the layers of molten ferrosilicon and liquidcalcium-silicate based slag. Typically, carbon monoxide and hydrogen aresupplied as reducing gases and argon and nitrogen are supplied as inertgases

By the method of the present invention it has surprisingly been foundthat practically 100% of the phosphorous in the calcium-silicate basedslag is transferred to the ferrosilicon alloy. Further, only a smallamount of iron is being transferred from the ferrosilicon alloy to thecalcium-silicate based slag.

By the method of the present invention it is thus possible to produce acalcium-silicate based slag having a phosphorous content of below 1 ppmwfrom a calcium-silicate slag which initially at least 30 ppmwphosphorous

Cheap sources of lime and quarts can thus be used to provide a highquality calcium-silicate based slag which is excellent for the removalof boron and phosphorous from molten silicon.

In one embodiment the calcium-silicate based slag treated for removal ofphosphorous is slag which has been used for slag treatment of moltensilicon and thus has an increased content of phosphorous. In this waycalcium-silicate based slag from treatment of molten silicon can beregenerated and recycled thus strongly reducing the costs for slagtreatment of molten silicon and avoiding disposal of large volumes ofused calcium-silicate based slag.

The method of the present invention can be carried out in conventionalhigh temperature furnaces such as induction furnaces and arc furnaces.

In a preferred embodiment it is used an arc furnace having verticalelectrodes and equipped with a lower tapping hole near its bottom and anupper tapping hole at a higher level. The process is started byestablishing a layer of molten ferrosilicon alloy at the bottom of thefurnace, where the top of the layer of ferrosilicon is well below theupper tapping hole. Thereafter slag forming compounds or used slag forregeneration is added until a layer of molten slag extending a distanceabove the upper tapping hole is formed. When the supply of slag formingcompounds is finished, the molten layer of slag is allowed to stay inthe furnace for a predetermined period to ensure removal of phosphorousfrom the slag and into the ferrosilicon alloy. The upper tapping hole isthen opened and the slag above the tapping hole is being tapped from thefurnace. After tapping, the upper tapping hole is closed and furtherslag forming compounds are supplied to the furnace. When the phosphorouscontent in the ferrosilicon alloy has increased to a preset value, theferrosilicon alloy is tapped from the lower tapping hole and the abovedescribed process is repeated. In this way it is obtained asemi-continuous, low cost production of low phosphorous calcium-silicatebased slag.

The present invention also relates to the use of the slag producedaccording to the method of the present invention for removal of boronand phosphorous from molten silicon.

DETAILED DESCRIPTION OF INVENTION EXAMPLE 1

A layer of molten ferrosilicon alloy consisting of 85% by weight Fe and15% by weight Si was provided at the bottom of an induction furnace. 120grams of quarts having a phosphorous content of 4 ppmw and 130 grams oflime having a phosphorous content of 35 ppm were added to the inductionfurnace and melted on the top of the molten ferrosilicon layer. Theinitial phosphorous content of the liquid slag was calculated to 20 ppmwbased on the phosphorous content of quarts and lime.

Three tests were run using different gas compositions for stirring themelt. The stirring gas was supplied through a graphite tube. Aftertreatment the slag was tapped from the induction furnace and analysedwith respect to phosphorous, boron, CaO, Si0₂ and Fe₂03. The results areshown in Table 1.

TABLE 1 Chemical Analysis Test P* B CaO SiO₂ Fe₂O₃ No. Stirring gas[ppmw] [ppmw] [wt %] [wt %] [wt %] A Ar—5% H₂ <2.5 8.1 53.44 46.48 0.12B Ar—25% CO <2.5 8.4 53.36 46.55 <0.12 C Ar <2.5 8.1 53.01 47.60 0.12*detection limit for P in slag: 2.5 ppmw (ICP)

The results in Table 1 show that the phosphorous content in the producedslags was below the detection limit of 2.5 ppm for all three slags.Based on calculations of mass balances from use of the produced slags inslag treatment of molten silicon, it was found that the phosphorouscontent of the three slags produced in fact was about 0.1 ppmw. Further,Table 1 shows that the content of Fe203 in the three slags was very low,indicating that only a minor amount of iron was transferred from theferrosilicon alloy to the slag phase.

EXAMPLE 2

The three slags A, Band C produced in Example 1 were used to removeboron and phosphorous from molten silicon. Molten metallurgical gradesilicon containing 47 ppmw boron, 9 ppmw phosphorous and 0.25% by weightof iron was treated with the slags A, B and C produced in Example 1. Theweight ratio of slag to silicon was 2.6 for all runs. The content ofphosphorous, boron and iron in the used slags and in the treated siliconwere analysed.

The results are shown in Table 2.

TABLE 2 Fe % by Pppmw Bppmw weight Slag in slag in Si in slag in Si InSi A 2.5 * 3 25.5 8.6 0.4 B 2.5 * 4 25.1 8.5 0.4 C 2.5 * 4 25.7 8.50.4 * detection limit for P in slag: 2.5 ppmw (ICP)

From the results in Table 2 it can be seen that a very good boronremoval was obtained for all three slags and that the phosphorouscontent in the treated silicon was reduced from 9 to about 4 ppmw.

1-5. (canceled)
 6. A method for producing a low phosphorouscalcium-silicate based slag for treatment of molten silicon in order toremove boron and phosphorous, characterized in that the moltencalcium-silicate based slag is treated with a molten ferrosilicon alloyin a vessel, whereby phosphorous in the calcium-silicate based slag istransferred to the ferrosilicon alloy, and removing a calcium-silicatebased slag containing less than 3 ppmw phosphorous from the vessel. 7.The method according to claim 6, characterized in that a molten layer ofthe ferrosilicon alloy is provided in the vessel whereafter a SiO₂source and a CaO source are supplied to the top of the layer offerrosilicon alloy to provide a layer of molten calcium-silicate basedslag, whereby phosphorous in the calcium-silicate based slag istransferred to the ferrosilicon alloy, and removing the low phosphorouscalcium-silicate based slag from the vessel
 8. The method according toclaim 6, characterized in that the molten ferrosilicon alloys isproduced in situ in the vessel by aiding Fe₃O₃ and Si together with thecalcium-silicate based slag.
 9. The method according to claim 6,characterized in that the ferrosilicon alloy contains up to 30% byweight of silicon, the remainder, except for normal impurities, beingiron.
 10. The method according to claim 9, characterized in that theferrosilicon alloy contains 10 to 20% by weight of silicon.
 11. Themethod according to claim 6, characterized in that the calcium-silicatebased slag supplied to the vessel is calcium-silicate based slag whichhas been used for slag treatment of molten silicon.
 12. The methodaccording to claim 6, characterized in that a reducing and/or an inertgas is supplied to the vessel in orders to stir the layers of moltenferrosilicon and liquid calcium-silicate based slag.
 13. The methodaccording to claim 12, characterized in that carbon monoxide andhydrogen are supplied as reducing gases.
 14. The method according toclaim 13, characterized in that argon and nitrogen are supplied as inertgases.
 15. Use of the calcium-silicate slag produced according to claim6 for slag treatment of molten silicon in order to remove boron andphosphorous.